Capacitor tantalum surface for use as a counterelectrode device and method

ABSTRACT

A counterelectrode and a method for providing the counterelectrode wherein an inherently high effective capacitance surface is formed on tantalum. The oxide forming ability of the tantalum surface is destroyed by removing existing oxide from the surface, depositing on the surface a non-continuous layer of a platinum family metal, and alloying the deposited metal with the tantalum thereby forming alloy layer. A second layer of metal, also selected from the platinum family, may then be deposited over the alloy layer. Alternately the platinum family metal may be sputtered onto the surface of the tantalum with or without an the alloying step. The second deposition produces a spongy layer and is accomplished by conventional electrolytic techniques.

BACKGROUND OF THE INVENTION

A. Field of the Invention

This invention relates to capacitors and in particular to anelectrolytic capacitor counterelectrode having a tantalum surface.

B. Background Art

The counterelectrode in a wet-type tantalum capacitor must present avery low impedance to an AC signal if the capacitance of the anodeelement is to be the determining factor in the capacitance of thefinished capacitor. This is generally accomplished by either the use ofa porous tantalum sintered body with high specific capacitance, as inthe all tantalum types or by deposition of a spongy layer of a member ofthe platinum group metals, as in the silver case types.

The ability to produce an effective surface on the case wall ofcapacitors where the inner surface of the case consists of tantalum isdesirable economically in parts using wholly tantalum cases. It is anecessity for laminated case parts where the nature of the laminatedstructure makes sinter or welding of a porous cathode body into the caseimpossible.

The ability of tantalum to form a thin, adherent, dielectric oxide isthe basis for tantalum capacitors but is also the primary problem inusing a tantalum surface for a counterelectrode. It is impossible, froma practical standpoint, to keep a tantalum surface oxide free and thespecific capacitance of an oxidized surface, either smooth or etched, isfar too low to make a satisfactory counterelectrode.

Furthermore, the presence of even the thinnest of air oxides precludesthe use of electrolytic or electroless deposition of a noble metal ashas traditionally been done in silver case parts. This is because theair oxide does not conduct electricity and deposition will only takeplace upon small breaks in the oxide.

A solution to this problem has been to find a method of treating thetantalum surface that destroys its ability to form an oxide. Any method,to be considered, must be physically and chemically compatible with allthe materials present in the case construction.

U.S. Pat. No. 4,523,255 issued Jan. 11, 1985 to Rogers teachesdestroying the oxide forming ability by converting a thin layer of thetantalum surface to tantalum carbide. This layer did not have any greateffective capacitance by itself but, more importantly, its nonmetallicnature limited the choices available for the required secondarytreatment. Rogers further taught the use of a carbon layer depositionfrom a colloidal dispersion of graphite. However, the graphite layer didnot adhere well to the tantalum carbide surface and in many ratingsrelied upon the addition of "depolarizing" agents to the capacitorelectrolyte.

U.S. Pat. No. 3,628,103 issued to Booe, disclosed forming on virtuallyany metal an inner layer of platinum on a cathode of a wet electrolytecapacitor. Tantalum in particular is not specified.

SUMMARY OF THE INVENTION

A counterelectrode and a method for providing the counterelectrodewherein an inherently high effective capacitance surface is formed ontantalum. The oxide forming ability of the tantalum surface is destroyedby forming a thin layer of an alloy of tantalum and one of the metals ofthe platinum family. The technique used in producing the alloy layer maybe one of the following: (1) the platinum family metal is deposited byelectroless plating after a suitable pretreatment of the tantalumsurface to remove the pre-existing oxide or (2) the platinum familymetal is deposited in a thin film by a sputtering process. The alloy isthen formed by a suitable heat treatment. In the sputtering technique,if the oxide on the tantalum surface is removed by glow discharge beforethe sputter step, the alloy is formed directly and the heat treatmentstep is then not necessary. Once the alloy layer is in place, thetreated cases, or cases drawn from stock so treated, can then have aspongy layer of a platinum family metal deposited on thecounterelectrode surface by conventional electrochemical methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. shows a flow chart representation of the method of the presentinvention for preparing a tantalum surface for use as acounterelectrode;

FIGS. 2A-2D show steps in the formation of the counterelectrode of thepresent invention.

FIG. 3 shows a cross-section of a typical of "wet slug" capacitor whichmay use the counterelectrode of FIGS. 2A-2D.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 there is shown method 10 for preparation of atantalum surface for use as a counterelectrode in tantalum "wet slug"capacitors. Method 10 may proceed by three paths, labelled 10a, 10b, and10c. FIGS. 2A-2D show the surface of a portion of tantalum 20 beingtreated according to method 10 of the present invention. Method 10 maybe used on stock from which cases are to be drawn. With appropriatefixturing and technique, method 10 may also be applied to the innersurface of already drawn cases or any other tantalum surface.

In block 11a the surface of tantalum 20 to be processed is treated witha solution containing a small amount of hydrofluoric acid to remove ordestroy the existing air oxide which has formed on the surface oftantalum 20. The surface of tantalum 20 is also treated with anactivator such as a stannous salt or an organic reducing agent. Theactivator reacts with the surface of tantalum 20, wets and cleans it,and provides seeding for adherence of materials required to destroy theoxidizing ability of the surface of tantalum 20.

In block 13a a non-continuous layer of small island-like structures 22of a member of the platinum group of metals is deposited upon thesurface of tantalum 20 using conventional electroless plating solutiontechniques as shown in FIG. 2B. Discontinuous sections or islands 22 areformed, rather than a continuous layer of the platinum group member,because, even after activation, it is extremely difficult to deposit aplatinum group member upon a tantalum surface using electroless platingmethods. Coverage of as little as 20% of the surface with islands 22 canbe sufficient.

The members of the platinum group which are believed to be suitable forthe capacitor counterelectrode of the present invention include Ru, Rh,Pd, Os, Ir, and Pt. It is also believed that of the members of theplatinum group, palladium is preferred because it results in the highestinherent capacitance and thereby provides the most efficient capacitor.

Deposit 22 is alloyed with the surface of tantalum 20 in block 15 byheat treatment thereby mixing the member of the platinum group with thetantalum at a molecular lever. The alloying step of block 15 forms alloylayer 24 as shown in FIG. 2C. The heat treatment of block 15 isperformed in a vacuum or in an inert atmosphere to prevent oxidation oftantalum 20. The temperature required to perform this alloying dependson which member of the platinum group is used. The lower temperaturelimit for each member of the platinum group is that temperature whichgives enough alloying to ensure adherence of the deposited platinumgroup member. This is required to destroy the ability of the tantalumsurface to form a dielectric oxide as previously described.

The upper temperature limit for each member is more difficult todetermine than the lower limit. It is believed that bulk alloying is notdesirable and that the temperature should not be raised to a level whichcauses bulk alloying. Furthermore, the temperature should be kept belowa level which would cause appreciable loss of material throughvaporization.

It is thus believed that a suitable temperature range for alloyingpalladium with tantalum may be about 825° to about 1400° C., foralloying platinum with tantalum a suitable temperature range may beabout 925° to about 1500° C. and that for alloying rhodium to tantalum asuitable temperature range may be about 975° to about 1650° C. It isalso believed that osmium and iridium require higher temperatures. Itwill be understood by those skilled in the art that alloyingtemperatures may be chosen appropriately for the specific combination ofmetals selected.

Additionally, these temperatures are limited by the type of materialsupon which method 10 is performed. For example, if method 10 isperformed upon tantalum 20 which is part of a clad of tantalum and someother metal the maximum temperature of alloying must be belowapproximately 1000° C. to prevent damage to the cladding bond.

When deposit 22 is alloyed with the surface of tantalum 20, an alloylayer 24, adhering to the surface of tantalum 20, is formed aspreviously described. The resulting alloy layer 24 may still bediscontinuous. The alloy surface may be composed of as little as 0.25atomic percent and still give satisfactory results. The surface of alloylayer 24 is suitable for electrodeposition of a member of the platinumgroup upon alloy layer 24. Thus, the surface of tantalum 20, which wasnot suitable for electrodeposition, is now covered with alloy layer 24,which is suitable for such deposition, thereby permitting the formationof a counterelectrode which has inherently high effective capacitance.

For the initial deposition 22 shown in FIG. 2B, the platinum groupmember may be sputtered onto the surface of tantalum 20 as shown inblocks 13b and 13c. It is believed that if the original oxide layer fromthe surface of tantalum 20 is removed, by glow discharge as shown inblock 11c of path 10c of method 10, the sputtering of block 13c mayresult in a mixing of the tantalum and the sputtered material at themolecular level thereby eliminating the alloying step of block 15 toform alloy layer 25 since the material sputtered onto tantalum 20 formslayer 24 if the sputtering energy is high enough. If the oxide layer isnot removed from the surface of tantalum 20 as shown in block 11c, thesputtered member of the platinum group forms deposits 22 as previouslydescribed and the alloying step of block 15 is required to form alloylayer 24 because the oxide layer remains between the sputtered materialand tantalum 20.

Additionally, it is believed that a tantalum-platinum group member alloymay be sputtered directly onto tantalum 20 thereby eliminating thealloying step of block 15.

After the alloying is complete and alloy layer 24 of tantalum withplatinum or a member of the platinum group is formed on tantalum 20, thecase is drawn in block 17 and a secondary layer 26 is formed as shown inblock 19. Secondary layer 26 may be any member of the platinum group andneed not necessarily be the same platinum group member that was alloyedwith tantalum 20 in alloy layer 24. Secondary layer 26 may be formedusing conventional electrolytic or electroless techniques since it mustadhere to platinum or platinum group member alloy layer 24 rather thanto a tantalum surface.

The deposition of secondary layer 26 shown in block 19 is performedunder conditions that produce a spongy physical structure. It is wellknown in the art how to choose electrical current levels duringdeposition and how to choose plating solutions in order to provide aspongy surface rather than a smooth surface for layer 26. The spongysurface is preferred because it provides more surface area forelectrical contact with an electrolyte.

Layers 24,26 are highly adherent to tantalum 20 and therefore layers24,26 are highly adherent to a case wall of a capacitor formed usingmethod 10. This eliminates the problem of conductive particles becomingfree and mixing in with the electrolyte and maintains the stability ofthe electrical properties of the counterelectrode.

Thus tantalum 20 is provided with a surface layer 24 which destroys theoxidizing ability of tantalum 20 wherein surface layer 24 over tantalum20 has an inherently high effective capacitance. Because of thisinherently high effective capacitance, capacitors produced with tantalum20 treated in accordance with method 10 have a reduced reliance ondepolarizing agents.

Referring to FIGS. 3 and 4, there is shown capacitor 50 which mayinclude a counterelectrode made according to method 10 of the presentinvention. It will be understood by those skilled in the art that acounterelectrode made in accordance with method 10 may be used in anyother type of wet slug capacitor and that capacitor 50 is merely used byway of illustration.

Capacitor 50 includes porous tantalum annode 38 which is mounted invibration spacer 40 in the bottom of can 42. The space between gasket 36and spacer 40 is filled with an electrolyte 37 which impregnates anode38. Leads 32,34 are electrically coupled to anode 38 andcounterelectrode can 42. Can 42 includes nickel or nickel alloy layer44, copper layer 46, and tantalum layer 20 as shown in magnified portion48. Tantalum layer 20 has been treated in accordance to method 10 of thepresent invention to provide alloy layer 24 and spongy secondary layer26.

Whether method 10 is performed using electroless deposition as shown inpath 10a of method 10 or sputtering as shown in paths 10b and 10c, allthe steps of method 10 may be used either on strip stock before drawingof a capacitor case such as case 42 or on the finished capacitor caseitself in which case the step of block 17 is not required. Thedeposition of secondary layer 26 onto the alloyed surface of layer 24however must be done on finished case 42 to prevent mechanical damage tolayer 26.

The choice between paths 10a,b,c is partly determined by cost. It isbelieved that the sputtering of paths 10b,c is preferred to thedisposition of path 10a. However, sputtering equipment involves a muchlarger initial cost. Avoiding the deposition of path 10a is preferred,for example, when processing strip stock because one side of strip stockmust be masked when chemically treating the other side.

Furthermore, method 10 may be performed upon full tantalum cases as wellas those having a laminated or clad structure with tantalum as the innerlayer such as case 42 having laminations including nickel layer 44 andcopper layer 46. If method 10 is used in making a clad can the layersare first bonded then sputtering or electroless deposition is performedand the alloying is performed but alloying is limited to one thousanddegrees, whether on a can or on strip stock, to prevent damage to thelaminations. When the alloying is performed the can is drawn as shown inblock 17. Method 10 may also be performed upon a counterelectrode foil(not shown) in a foil-type tantalum capacitor (not shown). The preferreduse for method 10 is in a triclad can such as can 42 of capacitor 50 inwhich a copper layer 46 is disposed between an inner tantalum layer 20and an outer nickel alloy layer 44. Most commonly the outer nickel alloylayer 44 is an alloy of nickel and copper.

It is claimed:
 1. A method for providing an inherently high effectivecapacitance counterelectrode surface on tantalum for use in anelectrolytic capacitor, comprising the steps of:(a) removing existingoxide from the tantalum; (b) depositing over the tantalum anoncontinuous layer of a first platinum family metal; and, (c) alloyingthe deposited first metal with the tantalum to form an alloy layer. 2.The method of claim 1 wherein step (c) is followed by the further stepof:(d) depositing over the alloy layer a layer of a second platinumfamily metal.
 3. The method of claim 2 wherein step (d) compriseselectrolytic plating over the alloy layer.
 4. The method of claim 2wherein the first metal is the same as the second metal.
 5. The methodof claim 2 wherein the first metal is different from the second metal.6. The counterelectrode of claim 5 wherein the tantalum surfacecomprises the surface of a solid tantalum can.
 7. The method of claim 1wherein step (a) comprises treating the surface of the tantalum with anacid.
 8. The method of claim 7 wherein the acid comprises hydrofluoricacid.
 9. The method of claim 1 wherein step (a) includes treating thesurface of the tantalum with an activator.
 10. The method of claim 1wherein step (b) comprises electroless plating the surface of thetantalum.
 11. The method of claim 1 wherein step (c) comprises providingheat in an inert atmosphere to the deposited first metal and tantalumsurface.
 12. The method of claim 11 wherein step (c) includes providingheat at a temperature between 825° and 165° C.
 13. The method of claim 1wherein the first metal is palladium.
 14. A counterelectrode for atantalum electrolytic capacitor having an inherently high effectivecapacitance inner tantalum surface, comprising an alloy first layerformed over the tantalum surface the alloy first layer comprisingtantalum and a platinum family metal.
 15. The counterelectrode of claim14 further comprising a second layer formed over the alloy first layerthe second layer comprising a platinum family metal.
 16. Thecounterelectrode of claim 15 wherein the first and second layerscomprise the same platinum family metal.
 17. The counterelectrode ofclaim 15 wherein the first and second layers comprise different platinumfamily metals.
 18. The counterelectrode of claim 15 wherein the secondlayer is spongy.
 19. The counterelectrode of claim 14 wherein thetantalum surface comprises the surface of a tantalum foil.
 20. Thecounterelectrode of claim 14 wherein the tantalum surface comprises atantalum layer surface on a laminated can.
 21. A method for providing aninherently high effective capacitance counterelectrode surface ontantalum for use in an electrolytic capacitor, comprising the stepsof:(a) removing existing oxide from the tantalum; (b) sputtering overthe tantalum a layer of a first platinum family metal.
 22. The method ofclaim 21 wherein step (b) is followed by the further step of:(c)depositing over the alloy layer a spongy layer of a second platinumfamily metal.
 23. The method of claim 22 wherein step (c) compriseselectrolytic deposition over the alloy layer.
 24. The method of claim 22wherein the first metal is the same as the second metal.
 25. The methodof claim 22 wherein the first metal is different from the second metal.26. The method of claim 21 wherein step (b) is followed by the step ofalloying the sputtered first metal to form an alloy layer.
 27. Themethod of claim 21 wherein step (a) comprises removing oxide by means ofa glow discharge process.
 28. The method of claim 21 wherein the firstmetal is palladium.